In 1909, the entomologist Carlo Emery noted that social parasites among insects (e.g., cleptoparasites) tend to be parasites of species or genera to which they are closely related.[1][2] Over the years, this pattern has been recognized in many additional cases, and generalized to what is now known as Emery's Rule.[1] The pattern is best known for various taxa of Hymenoptera.

The significance and general relevance of this pattern is still a matter of some debate, as a great many exceptions exist, though a common explanation for the phenomenon when it occurs is that the parasites may have started as facultative parasites within the host species itself (such forms of intraspecific parasitism are well-known, even in some subspecies of honeybees[3] ), but later became reproductively isolated and split off from the ancestral species, a form of sympatric speciation.